AFR question
01-19-2007, 10:02 AM
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AFR question
ok, i have a question regarding AFR. This as been discussed many times and i just want a serious answer. Every book i look in it says that the perfect AFR is 14.7, but people here are runnin 11's and 12's. Is this right? Does forced induction effect what you want as a Air/Fuel Rato? would really appreciate any help i could get.
01-19-2007, 10:08 AM
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The higher the number the more leaner and the lower more richer. On a F/I car the A/F is different then an N/A car. Yes your right about the 14.1 being good but thats on an N/A car. On a F/I car you don't wan't to go any thing higher than 12.5. 14.1 on a F/I car usualy means BOOM!!.
01-19-2007, 10:36 AM
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from my post for installation of the SAFC on ************** (https://www.cobaltss.net/forums/***************************/viewtopic.php?t=60):
Q: So who would want to put an SAFC on their car?
Well, anybody really. SAFC's are good for naturally/normally aspirated (n/a) cars as well as those that are boosted via turbo or supercharger (f/i, forced induction).
Q: Why would I want to put one on my car?
It's a quick answer really: performance. From the factory, cars come set to have an air to fuel ratio (AFR) at 14.7:1, this is called stoich which is known to be the best AFR for fuel economy. However, it's not the best one for performance. That AFR is 13.1:1. So, you can get your car at the dealership, slap this on it, and tune 'er for 13.1:1 AFR and actually squeeze a few more ponies out of it!
Since this is an air to fuel ratio, a lower number means there is less air in the engine and more fuel. This condition is known as being "rich" - the opposite is "lean". So the stoich mark is known for having just enough fuel in it so the engine can function, but little enough so that it's not being "wasted". When you move it to 13.1:1, you will notice a drop in MPG, but you'll feel it in your pants
Q: What should my AFR be if I'm boosted?
Well this is a twofold answer. It all depends on if you're building boost at the time. If you are not actually building boost, then your AFR can be sitting on "stoich" (for economy) or 13.1 (for good performance all the time). However when you're building boost, do not go any leaner (higher) than 12.5:1! The ecotec can handle slightly leaner amounts under boost, but you're really asking to blow up.
If you go leaner than that, you're risking a situation known as "detonation" or "knock" where your spark plug lights off the fuel before your piston has come around to accept the new charge and come back down. So you're basically exploding the fuel while the piston's still coming up - NOT GOOD! This is a very quick way to blow up your engine.
You can always go richer under boost (10, 11, 12:1 AFR) but you're sacrificing performance (and fuel! ). A recent stage kit swap on an SRT4 yielded nearly a 20hp gain going from mid-high 8:1 AFR to low 10:1 AFR. If the tune were to bring that out to 12's a nearly 60hp gain could be easily realized. So you want to go lean for more power, but not too lean so that you get knock.
Q: So who would want to put an SAFC on their car?
Well, anybody really. SAFC's are good for naturally/normally aspirated (n/a) cars as well as those that are boosted via turbo or supercharger (f/i, forced induction).
Q: Why would I want to put one on my car?
It's a quick answer really: performance. From the factory, cars come set to have an air to fuel ratio (AFR) at 14.7:1, this is called stoich which is known to be the best AFR for fuel economy. However, it's not the best one for performance. That AFR is 13.1:1. So, you can get your car at the dealership, slap this on it, and tune 'er for 13.1:1 AFR and actually squeeze a few more ponies out of it!
Since this is an air to fuel ratio, a lower number means there is less air in the engine and more fuel. This condition is known as being "rich" - the opposite is "lean". So the stoich mark is known for having just enough fuel in it so the engine can function, but little enough so that it's not being "wasted". When you move it to 13.1:1, you will notice a drop in MPG, but you'll feel it in your pants
Q: What should my AFR be if I'm boosted?
Well this is a twofold answer. It all depends on if you're building boost at the time. If you are not actually building boost, then your AFR can be sitting on "stoich" (for economy) or 13.1 (for good performance all the time). However when you're building boost, do not go any leaner (higher) than 12.5:1! The ecotec can handle slightly leaner amounts under boost, but you're really asking to blow up.
If you go leaner than that, you're risking a situation known as "detonation" or "knock" where your spark plug lights off the fuel before your piston has come around to accept the new charge and come back down. So you're basically exploding the fuel while the piston's still coming up - NOT GOOD! This is a very quick way to blow up your engine.
You can always go richer under boost (10, 11, 12:1 AFR) but you're sacrificing performance (and fuel! ). A recent stage kit swap on an SRT4 yielded nearly a 20hp gain going from mid-high 8:1 AFR to low 10:1 AFR. If the tune were to bring that out to 12's a nearly 60hp gain could be easily realized. So you want to go lean for more power, but not too lean so that you get knock.
01-20-2007, 10:59 AM
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I'll have to scan in the one from one of my books that shows the tradeoff in emissions with AFR as well.
Manufacturers target 14.7 afr for all driving conditions except wide open throttle because it is the ideal (or best comprimise) in the emissions.
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